*I asked the main trainer at my gym if he'd watch my form on my last set. He said my form looks good but says I'm squating a little to low and my rear is starting to tuck-under. I'm not sure how to tell or feel how i'm doing that.

That's not extremely uncommon. As long as your lower back is feeling ok I would be concerned too much. Just try not to worry too much about going below parallel and you'll be fine.